A common problem with very small craft such as canoes, kayaks, Jon boats, small speedboats, rigid inflatables, dinghies, and even larger small craft such as bass boats and bay boats has been what is commonly known as “tail dragging”. In some designs, this occurs at rest due to too much weight at the stern. In others, tail dragging occurs in operation, during acceleration, displacement mode operation (subplaning) or when on plane. In many designs, the problem arises in more than one mode of operation, sometimes at all times.
Another problem has been substantial weight increase to the point that even a small boat designed for use by one person can not be carried by one person as the weight, even without an efficient motor installed, generally grows to 100 pounds or more. A heavier hull is frequently used to balance a boat and give it more stability.
First, some background on the art, as it exists for lightweight craft that should be light enough for a single person to pick up and transport to and from the roof of a car to the water.
For the purposes of this invention and category, a light hull, heavy motor combination shall be defined as either of three conditions:
1) A motor installed anywhere in the hull of a boat that will cause the boat to tail drag or list to the rear, raising the bow, when in an unloaded or underloaded condition. This is without adding ballast to the hull.
2) A motor or motor package mounted toward the rear of a boat that exceeds 75% the weight of the boat without ballast.
3) A boat where the combination of desired fixed equipment, and or motor(s) added to the craft causes the boat to tail drag without adding ballast.
Solutions to the problem of using a heavy motor on a light craft have been numerous. One still common crude solution involves placing weights in the bow to compensate for the weight of a rear-mounted motor on a square stern kayak or canoe. Carrying an additional 50-60 pounds of weights to stuff into the bow is a less than elegant way to solve the problem of the bow of a canoe sticking up at as much as a 20° angle even after the operator gets in the boat in a typical position near the motor. Even with weight added, most canoes do not ride level and adding weight to the bow reduces cargo capacity. And if cargo is added in the bow to hold the bow down, it can be hazardous retrieving it.
Kayaks with motors attached at or near the stern of the craft tend to ride so tail heavy in the water that the stern is sometimes actually underwater. One common solution to that issue has been to mount a motor on one side of the canoe or kayak on a bar with a corresponding weight on the other side of the boat to compensate. This practice is usually limited to electric trolling motors because the added weight can become a problem if even a 30-40 pound 2 hp outboard motor is added. Even a 30 pound motor mounted in this way becomes a motor and mount package weighing at least 70 pounds. Most canoes and kayaks are not structurally designed to really take the weight and vibration of a motor, mount, and compensation weights that these devices would subject the boat to. In addition, installing any motor in this manner makes the boat difficult or impossible to paddle or row.
Another solution has been mounting a trolling motor, electric outboard, or ICE outboard engine amidships through a hole in the hull. Some of the commercial designs have incorporated a mount that allows the motor to lifted or tilted out of the water. So far, unless these use a specially designed motor head, draft is deeper than an outboard motor mounted in a more conventional manner. All of the electric systems have substantial draft. Disadvantages to these designs include reduced cargo capacity, deep draft compared to shallow drive portable outboard motors, weight, and performance that is usually not much faster than a good kayak, albeit, without the work.
Better results have been produced by kayak and canoe-like craft with an inboard power plant driving a propeller or a water jet drive. Some of these, such as the commercially available Surfango and Mokai powered kayaks offer very good performance and reasonable economy. They are single person boats. The Surfango Adventurer and similar boats are very high performance craft that do, in general, ride more bow high and “tail drag” more than would be desirable. The Surfango Adventurer still provides over 25 mph from only 9.5 hp due to its kayak-like hull. The Surfango weighs 120-130 pounds. The Mokai is a more practical craft that produces 14 mph from a 6 hp inboard jet pump. It has the advantage through the invention described in John Murray's U.S. Pat. No. 5,937,785, of having a removable inboard engine and water jet pump system. This didn't, contrary to the assertions in U.S. Pat. No. 5,937,785, make the commercially made boat light enough to be actually transported by one person without aid. The bare hull weight is still 100 pounds. That would be more than most people could carry down to the water, much less put on top of a car or van by themselves. The biggest problems with these craft are weight and the use of proprietary jet pump components that require the purchase of component replacement parts from a small manufacturer. This latter problem may limit the useful life of the product. While not a performance consideration of this patent, it is a convenience and marketing consideration of my design. The ability to replace virtually any attached part of the boat, including the drive system, with commonly available parts, combined with a durable, repairable hull enables a design that has a very long service life.
The weight of these inboard designs result in single person boats that are not transportable by one person.
Static tail dragging is also a problem in many other small craft when an outboard motor is placed in a conventional position at or near the stern of the boat. It is less common in boats with inboard engines due to the center of gravity being located several feet forward.
Common solutions to the problem include weight distribution changes such as ballast, or heavier hulls. Active ballast solutions such as water chambers in hulls which fill at rest and drain as the boat accelerates have been either proposed or used.
These solutions result in less performance, more time to achieve plane, less efficiency, and lower capacity for cargo.
Adjustable and non adjustable trim tabs have been used to adjust the fore-aft attitude of boats at speed since at least the 1930's. Trim devices on outboard engines have also been used for the same purpose in more recent years. Since these only effect the trim of boats while under forward movement, they are not directly useful to the purpose intended.
Rear sponsons or buoyant extensions have been incorporated into the stern of boats to help control the problem of tail dragging at rest or subplaning speeds. This is well established in the art.
U.S. Pat. No. 5,224,436 by John Stricker is a very inclusive patent that discloses a stern extension or sponson. This development differs significantly from my development in several respects.
1) Stricker's design (also U.S. Pat. Nos. 4,584,959; 6,000,357) is designed as a step at the rear portion of the hull to act as a buoyant structure at subplaning speeds when the hull is operating in displacement mode. At planing speeds, his stepped extension is intended to be ventilated as the hull rises on plane, having no contact with the water. Should water or air forces force the bow of the boat up, the extension sponsons would make contact with the water, slowing the boat and forcing the boat into a more level plane, at least once the ventilation was broken. A disadvantage of this is that the slowing produced as drag increases, followed by the re-acceleration as drag decreases can cause porpoising.
2) Stricker's tests on a 12 foot fiberglass boat with a beam of 3 feet indicate much lower efficiency and planing ability than my three 10 foot test craft, each with a 29 inch beam. Since longer hulls are generally considered more hydrodynamically efficient, a top speed of only roughly half of my tested speed with 4 hp used on both craft indicates a significant increase in efficiency for my design. My design also results in a solid cruising plane before Stricker's earlier design even reaches planing speed. Stricker's test boat had a tested total weight with driver of about 400 pounds and a total length of 12.75 feet. My test boat had a total loaded weight of about 320 pounds and a length of 10.25 feet. The length of his extensions were 9 inches. In my design the extensions are narrower, but 27 inches long. I experimented with short extensions and found their improvements to performance were substandard. A short 13 inch extension gave results similar to Stricker's. If I had had knowledge of Stricker's tests, I might not have even attempted this modification.
Stricker's development seems to be similar to the sponsons used on at least one model of commercially available large Jon boat.
Lindstrom's design presented in U.S. Pat. No. 4,813,365 presents a solution that includes a stern extension that follows the basic contours of the main hull in a tumblehome fashion. This is mostly presented for esthetic reasons. It does not depend upon a stepped rear extension, unlike most other uses of buoyant sponsons. It however, does depend upon reflex chines and a double deadrise hull structure for most of the hydrodynamic force.
The rear extension in Lindstrom's design calls for “an engine mounting system which positions an outboard engine astern”. It places the engine in the far astern mounting position on the back of the flotation chamber that serves as the combination engine mount. While this removes the outboard engine mounting well from the cabin of the boat, it still places the motor on the end of a pendulum, just like a normal outdrive, requiring a much heavier hull to compensate for the load at the very stern of the boat. The 2 foot long extension in U.S. Pat. No. 4,813,365 would not have enough buoyancy to completely support the weight of an appropriate outboard or inboard/outboard propulsion unit on the size hull shown in the drawings without significant draft, nor does it actually change the center of gravity or buoyancy other than simply lengthening the hull. It is really a motor mount box that is an extension that positions the engine in a normal outboard position.This brings us to the purposes of my invention.